Faster In-Network Evaluation of Spatial Aggregationin Sensor Networks

Spatial aggregation is an important class of queries for geoaware spatial sensor database applications. Given a set of spatial regions, it involves the aggregation of dynamic sensor readings over each of these regions simultaneously. Nested spatial aggregation involves one more level of aggregation, combining these aggregates into a single aggregate value. We show that spatial aggregate values can often be computed in-network, rather than waiting until the partial aggregate records reach the root as is now the case. This decreases the amount of communication involved in query evaluation, thereby reducing the network's power consumption. We describe an algorithm that allows us to determine when an aggregate record for any spatial region is ready to be evaluated in-network, based on decorating the routing tree with region leader lists. We also identify several important scenarios, such as nested spatial aggregation and filtering predicates, when the savings from our approach are expected to be particularly great.